1. Introduction
The Sakar–Strandzha zone (Bonchev, Reference Bonchev1903; Janichevski, Reference Janichevski1946; Pamir & Baykal, Reference Pamir and Baykal1947; Dimitrov, Reference Dimitrov1958; Aydin, Reference Aydin1974; Gocev, Reference Gocev1979, Reference Gocev1991; Dabovski & Zagorchev, Reference Dabovski and Zagorchev2009) is a major tectonic zone of the Alpine orogen in the northern Aegean region of the Eastern Mediterranean on the territories of Bulgaria and Turkey (Fig. 1, inset). It has been involved in pre-Alpine and Alpine tectonometamorphic events that have affected a Neoproterozic and mostly Palaeozoic high-grade metamorphic basement and its Triassic–Jurassic low-grade metasedimentary cover (Chatalov, Reference Chatalov1988, Reference Chatalov1990; Okay et al. Reference Okay, Satır, Tüysüz, Akyüz and Chen2001; Sunal et al. Reference Sunal, Satir, Natal’in, Topuz and Vonderschmidt2011; Bedi et al. Reference Bedi, Vasilev, Dabovski, Ergen, Okuyucu, Doǧan, Tekin, Ivanova, Boncheva, Lakova, Sachanski, Kuşcu, Tuncay, Demiray, Soycan and Göncüoglu2013; Natal’in et al. Reference Natal’in, Sunal, Gun, Wang and Zhiking2016) (Fig. 1). Stratigraphic and tectonic relationships within the Sakar–Strandzha zone nappe pile, that is, the N-wards thrusting of the Triassic metasedimentary rocks onto both the Middle Jurassic (Bathonian) metasedimentary rocks and unmetamorphosed Upper Cretaceous (Cenomanian) sedimentary rocks (Chatalov, Reference Chatalov1988, Reference Chatalov1990), provided evidence for the Alpine tectonic and metamorphic history of the zone. U–Pb zircon dating of igneous and/or meta-igneous bodies and host metamorphic rocks in the Sakar–Strandzha zone revealed protracted Palaeozoic magmatic crystallization (Okay et al. Reference Okay, Satır, Tüysüz, Akyüz and Chen2001; Sunal et al. Reference Sunal, Natal’in, Satir and Toraman2006; Natal’in et al. Reference Natal’in, Sunal, Satir and Toraman2012; Georgiev et al. Reference Georgiev, von Quadt, Heinrich, Peytcheva and Marchev2012; Machev et al. Reference Machev, Ganev and Klain2015; Bonev et al. Reference Bonev, Filipov, Raicheva and Moritz2019 a), and also provided some information about early and late Palaeozoic metamorphic history (Okay et al. Reference Okay, Satır, Tüysüz, Akyüz and Chen2001; Natal’in et al. Reference Natal’in, Sunal, Gun, Wang and Zhiking2016; Bonev et al. Reference Bonev, Filipov, Raicheva and Moritz2019 a), which was overprinted by Alpine metamorphism. The latter, which occured during Bathonian–Cenomanian time according to the stratigraphy, is also indicated by the same temporal spread of the available dates of mid- to low-temperature radioisotopic systems (Fig. 1).
Synthetic geological map of the Sakar–Strandzha zone in Bulgaria and Turkey (modified after Bonev et al. Reference Bonev, Filipov, Raicheva and Moritz2019 a, b). Inset: Tectonic framework of the Alpine orogenic system in the northern Aegean region of the eastern Mediterranean domain. Geochronology: normal-font numbers: K–Ar ages (Boyadzhiev & Lilov, Reference Boyadzhiev and Lilov1972; Palshin et al. Reference Palshin, Skenderov, Bozkov, Mihailov, Kotov, Bedrinov and Ivanov1989); bold: K–Ar ages (Lilov et al. Reference Lilov, Maliakov and Balogh2004); italics: 40Ar/39Ar ages (Elmas et al. Reference Elmas, Yilmaz, Yigitbas and Ulrich2010); bold and italics: 40Ar/39Ar ages (Bonev et al. Reference Bonev, Spikings, Moritz and Marchev2010).
Late Palaeozoic metamorphism in the Sakar–Strandzha zone was initially demonstrated by a whole-rock Rb–Sr age of 244 ± 11 Ma for the Kirklareli metagranite (see Fig. 1) (Aydin, Reference Aydin1974) at the Palaeozoic–Mesozoic boundary. U–Pb zircon dating in both the metagranites and the host high-grade basement gneisses, together with overlaping late Carboniferous – early Permian ages and common metamorphic fabric, was used as an argument for early Permian upper greenschist to lower amphibolite-facies metamorphism at c. 271 Ma linked to the Variscan orogeny (Okay et al. Reference Okay, Satır, Tüysüz, Akyüz and Chen2001). Lower amphibolite-facies metamorphism at the Carboniferous–Permian boundary was shown as the most obvious from the textures, field and U–Pb zircon geochronologic data for the Sakar–Strandzha zone metagranites and the host basement gneisses (Sunal et al. Reference Sunal, Satir, Natal’in, Topuz and Vonderschmidt2011; Machev et al. Reference Machev, Ganev and Klain2015; Bonev et al. Reference Bonev, Filipov, Raicheva and Moritz2019 a). A pre-Middle Ordovician metamorphic event (c. < 484–450 Ma) was also inferred from the crystallization age of a magmatic body (Bonev et al. Reference Bonev, Filipov, Raicheva and Moritz2019 a) and the detrital zircons contained in the high-grade basement and cover metasedimentary rocks (Sunal et al. Reference Sunal, Satir, Natal’in and Toraman2008; Bonev et al. Reference Bonev, Filipov, Raicheva and Moritz2019 b), which might extend to the Neoproterozoic–Cambrian (Natal’in et al. Reference Natal’in, Sunal, Gun, Wang and Zhiking2016).
Alpine metamorphism, which reaches greenschist facies in the metagranitoids (i.e. the Kirklareli metagranite), was revealed by a dark mica and whole-rock Rb–Sr isochron date of 155 ± 2 Ma, which was connected to N-directed thrusting related to middle Mesozoic orogeny (Okay et al. Reference Okay, Satır, Tüysüz, Akyüz and Chen2001). Upper greenschist- to epidote-amphibolite-facies metamorphism at a temperature (T) of 485–530°C and pressure (P) of 6–8 kbar is documented in the metagranitoids and the host basement gneisses, amphibolites and schists, and yields Rb–Sr muscovite and biotite dates spanning 163–134 Ma (Sunal et al. Reference Sunal, Satir, Natal’in, Topuz and Vonderschmidt2011). The latter authors have interpreted these early Alpine metamorphic dates in a compressional, N-verging nappe tectonic context.
K–Ar ages of illite-muscovite in very-low-grade Jurassic phyllites and low-grade Triassic and Palaeozoic phyllites (T = 350–450°C; P = 3–5 kbar) in the Bulgarian part of the Sakar–Strandzha zone range from 118 Ma to 173 Ma, with an age of 123 Ma for the high-grade basement gneiss (Lilov et al. Reference Lilov, Maliakov and Balogh2004). A dark mica K–Ar age of 133 Ma was also reported by Firsov (Reference Firsov1975) for the high-grade basement schist. Lilov et al. (Reference Lilov, Maliakov and Balogh2004) have interpreted the K–Ar ages as recording Jurassic (160–170 Ma) (i.e. early Alpine) regional greenschist-facies metamorphism and subsequent resetting of the K–Ar system, yielding ages between 89 Ma and 128 Ma. Granitoids (i.e. Permian Sakar batholith) that intrude the high-grade metamorphic basement yield K–Ar dark mica ages that range from 100 Ma to 141 Ma (Boyadzhiev & Lilov, Reference Boyadzhiev and Lilov1972; Palshin et al. Reference Palshin, Skenderov, Bozkov, Mihailov, Kotov, Bedrinov and Ivanov1989) (Fig. 1).
40Ar/39Ar white and dark mica ages in the eastern Turkish part of the Sakar–Strandzha zone encompass 119–152 Ma interval in the metagranitoids, 123–138 Ma in the high-grade basement and 143–152 Ma in the Triassic–Jurassic metasedimentary rocks (Elmas et al. Reference Elmas, Yilmaz, Yigitbas and Ulrich2010) (see Fig. 1), interpreted to date cooling of the Alpine metamorphism and exhumation of the metamorphic pile in an extensional tectonic context. Neubauer et al. (Reference Neubauer, Bilyarski, Genser, Ivanov, Peytcheva and von Quadt2010) report 40Ar/39Ar ages of amphibole and white mica ranging over 136–144 Ma in the Sakar unit of the Sakar–Strandzha zone (see Fig. 1), which they interpret to date ductile, mylonitic deformation along the contact of the Sakar batholith and the host high-grade basement metamorphic rocks. According to Neubauer et al. (Reference Neubauer, Bilyarski, Genser, Ivanov, Peytcheva and von Quadt2010), the white mica age decreased to c. 124 Ma further south in the basement metamorphic rocks, due to an initial thermal overprint and a subsequent overprint at c. 69 Ma. Recently, hydrothermal activity in the Sakar batholith was bracketed between 149 ± 7 Ma and 114 ± 1 Ma by U–Pb apatite and titanite dates, respectively (Szopa et al. Reference Szopa, Salacinska, Gumsley, Shew, Petrov, Gaweda, Zagorska, Deput, Gospodinov and Banasik2020), which questions temporal relationships between Alpine metamorphism and hydrothermal activity. Finally, early Alpine metamorphism in the Sakar–Strandzha zone was succeeded by erosion-driven, slow-cooling-exhumation during Late Cretaceous – Eocene time, as derived from the apatite fission-track data (Cattò et al. Reference Cattò, Cavazza, Zattin and Okay2017).
More precise age constraints for the protracted Alpine metamorphism (89–173 Ma) of the Sakar–Strandzha zone are critical to improve our understanding of the timing of its tectonometamorphic history, which also requires an assessment of the regional-scale links. Here, we apply 40Ar/39Ar dating of metamorphic rocks in the Sakar unit of the Sakar–Strandzha zone in Bulgaria (Figs 1, 2), with the aim of constraining more precisely the timing of early Alpine metamorphism, which is highly relevant to the Mesozoic evolution of the Alpine orogen in the northern Aegean region.
Geological map of the Sakar unit (compiled after Kozhoukharova & Kozhoukharov, Reference Kozhoukharova and Kozhoukharov1973; Savov & Dabovski, Reference Savov, Dabovski, Kozhoukharov and Dabovski1980; Chatalov, Reference Chatalov1992; Dabovski et al. Reference Dabovski, Savov, Chatalov and Shiliafov1994) showing the locations of the samples studied for 40Ar/39Ar geochronology. Age results for the geologic subunits are incorporated in the map after Bonev et al. (Reference Bonev, Filipov, Raicheva and Moritz2019 a, b, c).
2. Geological outline of the Sakar unit
The pre-Upper Cretaceous geology of the Sakar unit is dominated by felsic igneous and meta-igneous bodies and high-grade country metamorphic basement rocks, which are overlain by Triassic metasedimentary rocks. They are subdivided into five subunits that include (Fig. 2): (i) a variegated complex of the high-grade basement equated to the Rhodope Massif high-grade basement (Kozhoukharov, Reference Kozhoukharov1987); (ii) the Lesovo gneiss-granite complex (Boyanov et al. Reference Boyanov, Kozhoukharov and Savov1965) and the intruded leucocratic dyke and stock granitoid bodies; (iii) the Melnitsa orthometamorphic complex (Chatalov, Reference Chatalov1992); (iv) the Sakar granitoid batholith (Boyanov et al. Reference Boyanov, Kozhoukharov and Savov1965; Dabovski & Haydoutov Reference Dabovski and Haydoutov1980; Kamenov et al. Reference Kamenov, Vergilov, Dabovski, Vergilov and Ivchinova2010); and (v) the Sakar-type metasedimentary rocks of the Lower–Middle Triassic Topolovgrad Group (Chatalov, Reference Chatalov1990, Reference Chatalov1991). The Upper Cretaceous – Quaternary subunits are beyond the scope of the paper.
The variegated complex consists of intercalated schist, gneiss and amphibolite of inferred Precambrian age (e.g. Kozhoukharov, Reference Kozhoukharov1987). Similar high-grade basement lithologies in Turkey yielded Proterozoic–Permian (2450–271 Ma) detrital zircon dates (Sunal et al. Reference Sunal, Satir, Natal’in and Toraman2008; Natal’in et al. Reference Natal’in, Sunal, Satir and Toraman2012). Fossil evidence indicates uppermost Ordovician – middle Silurian (Lakova et al. Reference Lakova, Gocev and Yanev1992), Lower Devonian (Zacharieva-Kovacheva et al. Reference Zacharieva-Kovacheva, Ware and Chatalov1964; Malyakov & Prokop, Reference Malyakov and Prokop1997) and lower Permian (Malyakov & Bakalova, Reference Malyakov and Bakalova1978) protoliths of the crystalline basement schist in the Sakar–Strandzha nappe stack. Tschermakite-bearing amphibolites in the variegated complex have island-arc basalt protoliths and record metamorphic conditions of T = 625–635°C and P = 6.5–8 kbar (Chavdarova & Machev, Reference Chavdarova and Machev2017). The same amphibole composition as the amphibolites of the same subunit in Turkey is considered to have recrystallized during the latest metamorphic event of Alpine age (Sunal et al. Reference Sunal, Satir, Natal’in, Topuz and Vonderschmidt2011).
The Lesovo gneiss-granite complex consists mostly of texturally distinct gneiss of granodioritic-dioritic composition (Boyanov et al. Reference Boyanov, Kozhoukharov and Savov1965; Kozhoukharova & Kozhoukharov, Reference Kozhoukharova and Kozhoukharov1973), which has a latest Carboniferous protolith age of 305.8 ± 1.4 Ma (Bonev et al. Reference Bonev, Filipov, Raicheva and Moritz2019 a). E–W-oriented leucocratic dykes and stock-like bodies cross-cut the variegated complex and the Lesovo complex. A stock-like body crystallized at the eastern margin of the Sakar batholith during Middle Ordovician time (461.6 ± 2.7 Ma), whereas a leucocratic dyke exposed at the western margin of the batholith has a Middle Triassic crystallization age (242.1 ± 1.8 Ma, Bonev et al. Reference Bonev, Filipov, Raicheva and Moritz2019 b).
The Melnitsa orthometamorphic complex (e.g. Chatalov, Reference Chatalov1992) is a volcano-plutonic complex that consists of calc-alkaline porphyritic metagranite, (meta)granite-porphyries and (meta)rhyolite located east of the Sakar batholith and within the Lesovo complex (Fig. 2). The porphyritic metagranite and the (meta)rhyolite of the Melnitsa complex crystallized at 300.2 ± 3.4 Ma and 297.2 ± 4.6 Ma, respectively (Bonev et al. Reference Bonev, Filipov, Raicheva and Moritz2019 a).
The undeformed and unmetamorphosed Sakar granitoid batholith is an E–W-aligned elongated dome-like body (Fig. 2). It mainly consists of calc-alkaline K-feldspar porphyritic granite and equigranular granite types (Dabovski & Haydoutov, Reference Dabovski and Haydoutov1980; Kamenov et al. Reference Kamenov, Vergilov, Dabovski, Vergilov and Ivchinova2010), which crystallized at 296.1 ± 2.7 Ma and 295.3 ± 1.9 Ma, respectively (Bonev et al. Reference Bonev, Filipov, Raicheva and Moritz2019 a).
As the Sakar batholith contains well-foliated and folded gneiss, schist and amphibolite xenoliths from the variegated complex, this feature indicates a pre-Permian high-grade metamorphism and deformation. Both processes are also confirmed by the Lesovo complex that contains gneiss and schist xenoliths. The Melnitsa complex is less deformed and less metamorphosed compared with all other mentioned metamorphic basement subunits.
The Topolovgrad Group unconformably overlies the variegated complex and consists of Triassic metasedimentary rocks that have been subdivided into three formations in the following stratigraphic order (Chatalov, Reference Chatalov1990, Reference Chatalov1991). The lower clastic Paleokastro Formation passes into the clastic-carbonate Ustrem Formation, whose upper levels contain latest Early Triassic bivalves, which is in turn overlain by the Middle Triassic, conodont-bearing carbonate Srem Formation. The Sakar batholith and the Melnitsa complex magmatic rocks have been reworked as pebbles and clasts in the sedimentary fill of the Paleokastro Formation (Boyanov et al. Reference Boyanov, Kozhoukharov and Savov1965; Dabovski & Haydoutov, Reference Dabovski and Haydoutov1980; Chatalov, Reference Chatalov1990, Reference Chatalov1991, Reference Chatalov1992). U–Pb detrital zircon geochronology from the base of the Paleokastro Formation up to the middle stratigraphic levels of the Ustrem Formation revealed a late Permian maximum depositional age of 259.1 ± 5.8 Ma (Bonev et al. Reference Bonev, Filipov, Raicheva and Moritz2019 c). The Topolovgrad Group is interpreted as a fluvial–alluvial succession, starting with the basal Paleokastro Formation that progressively fines upwards into the Ustrem Formation, passing upwards into a shallow-water carbonate platform of the Srem Formation, all of which were deposited in a Triassic sedimentary basin (Chatalov, Reference Chatalov1991). A superimposed metamorphic conformity was proposed for the position of the Paleokastro Formation onto the variegated complex (Savov & Dabovski, Reference Savov, Dabovski, Kozhoukharov and Dabovski1980) and onto the Melnitsa complex (Chatalov, Reference Chatalov1992). The metamorphic grade of the Topolovgrad Group reaches a low-temperature amphibolite facies, with muscovite–biotite–almandine garnet ± staurolite ± kyanite, which defines metamorphic conditions of T = 520–550°C and P = 2.5–9 kbar for the metapelitic rocks of the Ustrem Formation (Chatalov, Reference Chatalov1990). Chavdarova & Machev (Reference Chavdarova and Machev2017) have reported metamorphic conditions of T = 510–555°C and P = 6–8 kbar in the amphibolites of the Ustrem Formation. Quartz–biotite–muscovite schist intercalations are present in metaconglomerate and arkosic metasandstone, which are the most common rock types of the Paleokastro Formation. The Ustrem Formation has an irregular alternation of the main rock types of quartz–mica schist with biotite and garnet ± staurolite porphyroblasts, marbles, quartz–mica–calcite schist with biotite porphyroblasts, and subordinate metasandstone and amphibolite. The Srem Formation consists of calcite and dolomite marbles. The metamorphism associated with a weak deformation progressively decreases up the stratigraphic section of the Topolovgrad Group. The metamorphism of the Topolovgrad Group is related to the N-verging nappe-staking event either at the end of the Triassic Period (i.e. based on the age of the Srem Formation) or during post-Middle Jurassic time (i.e. based on the youngest Jurassic rocks in the Sakar–Strandzha zone).
3. Samples and 40Ar/39Ar results
We focused on key lithologies in the subunits of the Sakar unit, which include an amphibolite from the variegated complex of the high-grade basement and a metasandstone from the Paleokastro Formation of the Topolovgrad Group. Sample numbers mentioned below refer to sample locations depicted in Figure 2, which are given with their coordinates in online Supplementary Table S1 (available at http://journals.cambridge.org/geo).
Field observations of the high-grade basement south of the Sakar batholith revealed a kilometre-thick and regional foliation-parallel amphibolite sequence within the variegated complex, located to the west of the village of Dervishka mogila (Fig. 2). This amphibolite sequence might represent an original mafic sill-like body emplaced in a sedimentary succession, and then both metamorphosed to amphibolite facies. The fine- to medium-grained amphibolite (sample S53) mainly consists of amphibole, quartz and plagioclase (Fig. 3a). Subordinate metamorphic mineral phases are epidote, ilmenite and biotite. Prismatic to elongated needle-like green amphibole is inter-grown with the later mineral phases. Accessories include zircon and apatite. The amphibole defines the foliation both macro- and microscopically, and the quartz is slightly recrystallized.
Microphotographs of the samples used for 40Ar/39Ar geochronology in the Sakar unit. (a) Amphibolite (sample S53) from the variegated complex of the high-grade metamorphic basement showing recrystallization under amphibolite-facies conditions, plane-polarized light. (b) Two-mica schist (sample S10) from the Paleokastro Formation, cross-polarized light. am – amphibole; dm – dark mica; ep – epidote; afs – alkali feldspar; qtz – quartz; wm – white mica; pl – plagioclase.
Metasandstone sample S10 of the Paleokastro Formation was sampled from its uppermost stratigraphic level just below the first quartz–chlorite–muscovite schist with biotite porphyroblasts of the Ustrem Formation (Fig. 2). The coarse-grained metasandstone contains rare quartz clasts that locally delineate thin discontinuous layers of matrix-supported metaconglomerate. Metasandstone S10 is a two-mica schist consisting of quartz, alkali feldspar, and dark and white mica in modally decreasing abundances. Quartz is moderately sorted, showing large mono- and polycrystalline grains, the smaller of which are recrystallized. Alkali feldspar has the same sorting and is perthitic microcline. Brown-green dark mica forms large flakes of different orientation relative to the foliation. White mica mostly defines a discontinuous foliation that anastomoses the clastic grains. Micas overgrew these grains or grew in their interstices. Magnetite grains and aggregates and rare quartz–feldspar lithic fragments also occur. Accessory minerals include apatite, zircon and rutile.
Amphibole in amphibolite S53 and white mica in metasandstone S10 were separated for 40Ar/39Ar dating from a 250–400-μm sieve fraction using conventional magnetic and heavy liquid methods. The mineral concentrates were purified by hand-picking under a binocular microscope. 40Ar/39Ar analyses were conducted at the University of Geneva using an Argus VI (Thermo Fischer Scientific) multi-collector mass spectrometer. 40Ar was collected using a Faraday collector with a feedback resistance of 1 × 1012 Ohms, whereas 39–36Ar was collected on a Faraday collector with a feedback resistance of 1 × 1013 Ohms. Analytical details and procedures are as described in Villagómez & Spikings (Reference Villagómez and Spikings2013). All dates were calculated from blank- and baseline-corrected measurements, while mass discrimination was monitored by analyses of air aliquots after every 10 heating steps (online Supplementary Table S1). Neutron irradiation was monitored using Fish Canyon Tuff Sanidine, with a 40Ar/39Ar age of 28.201 ± 0.046 Ma (Kuiper et al. Reference Kuiper, Deino, Hilgen, Krijgsman, Renne and Wijbrans2008). Gas was liberated from the samples using an infrared laser (Photon Machines Inc.), and cleaned in an ultra-high vacuum steel extraction line equipped with a cold finger at −132°C and a GP50 (ST101) getter. The 40Ar/39Ar age spectra and 36Ar/40Ar to 39Ar/40Ar isochron ages are shown in Figure 4, all with a corresponding 2σ analytical error.
40Ar/39Ar geochronology results for the samples S53 and S10 from the Sakar unit. (a, b) 40Ar/39Ar age spectra and (c, d) 36Ar/40Ar to 39Ar/40Ar isochron ages. For the sample locations see Figure 2.
Amphibole from amphibolite S53 yields a 40Ar/39Ar plateau date of 140.50 ± 1.75 Ma from 100% of the 39Ar released (in 10 steps; Fig. 4a), which overlaps with the inverse isochron date of 140.91 ± 2.11 Ma (Fig. 4c). White mica from metasandstone S10 gave a plateau 40Ar/39Ar date of 126.19 ± 1.29 Ma from nine steps (90% of the total 39Ar released), which overlaps with the inverse isochron age of 127.02 ± 1.72 Ma (Fig. 4b, d).
4. Discussion
The mineral assemblages and textures in the dated samples show that the white mica and amphibole recrystallized during amphibolite-facies metamorphism, at temperatures above their closure temperatures range for argon in white mica (c. 440°C, Harrison et al. Reference Harrison, Célérier, Aikman, Hermann and Heizler2009) and in amphibole (535 ± 50°C) (e.g. Harrison, Reference Harrison1981; McDougall & Harrison, Reference McDougall and Harrison1999). The new 40Ar/39Ar ages therefore determine the cooling histories of the dated metamorphic rocks within 535–440°C, after the peak of the regional amphibolite-facies metamorphism. This temperature window coincides with greenschist-facies metamorphism within the crust, and therefore constrains the timing of the transition to such conditions of the metamorphic pile.
The 40Ar/39Ar date of 140.50 ± 1.75 Ma of amphibolite S53 from the variegated complex, which constitutes the country rocks of the Sakar batholith, indicates that the high-grade metamorphic basement cooled below c. 500°C during earliest Cretaceous time. The amphibole defines the planar metamorphic fabric in the rock (Fig. 3a) and shows that the cooling age that developed during Ar closure in this mineral testifies to the end of regional amphibolite-facies metamorphism. The white mica date of metasandstone S10 indicates that the Paleokastro Formation cooled below c. 440–400°C at 126.19 ± 1.29 Ma, subsequent to amphibolite-facies metamorphism, when the temperatures regressed to greenschist-facies conditions. This interpretation fits the regional time scale provided by the K–Ar and 40Ar/39Ar mica dates for the cooling stage towards greenschist-facies conditions in the Sakar–Strandzha zone of 173–100 Ma (see Fig. 1), subsequent to epidote-amphibolite-facies metamorphism at 163–134 Ma (Sunal et al. Reference Sunal, Satir, Natal’in, Topuz and Vonderschmidt2011) or c. 155 Ma (Okay et al. Reference Okay, Satır, Tüysüz, Akyüz and Chen2001) as derived from the Rb–Sr ages.
40Ar/39Ar geochronologic results for the Sakar unit should be considered within a broad regional context, including the adjacent Rhodope Massif, located just to the south of this unit (Fig. 1). This is based on the recent geochronologic record of Middle Triassic magmatism on both sides of the Maritsa river valley, which demonstrates the extension of the Sakar unit up to the limit of the eastern Rhodope Massif (Bonev et al. Reference Bonev, Filipov, Raicheva and Moritz2019 b; see Fig. 1).
There are field, textural and U–Pb zircon geochronologic evidence for Late Jurassic amphibolite-facies metamorphism at 160–154 Ma (Bonev et al. Reference Bonev, Marchev, Moritz and Collings2015), or high-pressure metamorphism at 158–150 Ma (Liati et al. Reference Liati, Gebauer, Fanning, Dobrzhinetskaya, Faryad, Wallis and Cuthbert2011, Reference Liati, Theye, Fanning, Gebauer and Rainer2016) in the eastern Rhodope Massif. The amphibolite-facies metamorphism is temporarily connected with the upper greenschist-facies metamorphism at 157–154 Ma, which is coeval with the emplacement of the Kulidzhik nappe from the Circum–Rhodope belt on top of the Rhodope metamorphic pile, as determined using 40Ar/39Ar geochronology (Bonev et al. Reference Bonev, Spikings, Moritz and Marchev2010). The overlap of the K–Ar and 40Ar/39Ar ages of the Sakar–Strandzha zone and the 40Ar/39Ar ages of the Circum–Rhodope belt in the eastern Rhodope Massif (see Fig. 1) indicate that cooling of amphibolite-facies metamorphism and the thermal transition to upper greenschist metamorphism was a common event for both areas. A similar early Mesozoic stratigraphy of the Sakar–Strandzha zone and the Circum–Rhodope belt in the eastern Rhodope Massif was emphasized due to their tight facial and temporal relationships in a single zone linked to common N-wards nappe emplacement (Gocev, Reference Gocev1979, Reference Gocev1991; Chatalov Reference Chatalov1988, Reference Chatalov1990). Subsequent studies in the Sakar–Strandzha zone and the Circum–Rhodope belt have confirmed the metamorphic grade and temporal spread of Late Jurassic – Early Cretaceous thrust tectonics (Okay et al. Reference Okay, Satır, Tüysüz, Akyüz and Chen2001; Bonev & Stampfli, Reference Bonev and Stampfli2011; Sunal et al. Reference Sunal, Satir, Natal’in, Topuz and Vonderschmidt2011; Bonev et al. Reference Bonev, Marchev, Moritz and Collings2015; Natal’in et al. Reference Natal’in, Sunal, Gun, Wang and Zhiking2016). The new 40Ar/39Ar ages obtained for the Sakar unit further confirm regional-scale correlations between the eastern Rhodope Massif and the Sakar–Strandzha zone with respect to the early Alpine tectonic and metamorphic evolution, providing evidence for Early Cretaceous cooling after amphibolite-facies metamorphism.
5. Conclusions
Our 40Ar/39Ar isotopic study shows that early Alpine regional amphibolite-facies metamorphism of the Sakar unit cooled through c. 550–440°C during Early Cretaceous time (c. 141–126 Ma) towards greenschist-facies conditions. Time constraints for cooling retrogression from the amphibolite-facies metamorphism, which initiated during Late Jurassic time in the Sakar–Strandzha zone, reveal an equivalent temporal metamorphic history with amphibolite- to greenschist-facies metamorphism of the Circum–Rhodope belt, and common Late Jurassic – Early Cretaceous regional-scale thrust tectonics.
Supplementary material
To view supplementary material for this article, please visit https://doi.org/10.1017/S0016756820000953
Acknowledgements
The study was supported by the Bulgarian National Science Fund project no. DN 04/6. We thank both anonymous reviewers for their comments, as well as the editorial comments, which helped us to improve the paper.
